Moving laser beam readers or laser scanners, as well as solid-state imaging systems or imaging readers, have both been used, both in a handheld and hands-free modes of operation, to electro-optically read one-dimensional bar code symbols, particularly of the Universal Product Code (UPC) type, each having a row of bars and spaces spaced apart along one direction, and two-dimensional symbols, such as Code 49, which introduced the concept of vertically stacking a plurality of rows of bar and space patterns in a single symbol, as described in U.S. Pat. No. 4,794,239. Another two-dimensional code structure for increasing the amount of data that can be represented or stored on a given amount of surface area is known as PDF417 and is described in U.S. Pat. No. 5,304,786.
The known moving laser beam reader generally includes a laser scan engine or module for supporting an electrically energizable laser for emitting a laser beam, a focusing lens assembly for focusing the laser beam to form a beam spot having a certain size at a focal plane in a range of working distances, an electrically energizable scan component for repetitively scanning the beam spot across a symbol in a scan pattern, for example, a scan line or a series of scan lines, across the target symbol multiple times per second, e.g., forty times per second, a photodetector for detecting laser light reflected and/or scattered from the symbol and for converting the detected laser light into an analog electrical signal, and electrical signal processing circuitry including a digitizer for digitizing the analog signal. Sometimes, the laser scan engine also supports a controller or microprocessor for controlling operation of the electrical components supported by the laser scan engine, and for decoding the digitized signal based upon a specific symbology used for the symbol.
The known imaging reader includes an imaging scan engine or module for supporting a solid-state, image sensor comprising an array of pixels or light sensors, for sensing return light from a target being imaged. The image sensor may be a one- or two-dimensional charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) device, and is analogous to the image sensors used in electronic digital cameras. The target can be a symbol or a non-symbol. The imaging scan engine also supports an illuminating light assembly for illuminating the symbol with illumination light from an illumination light source, e.g., one or more light emitting diodes (LEDs); an imaging lens assembly, e.g., one or more imaging lenses, for capturing return ambient and/or illumination light scattered and/or reflected from the symbol over a field of view and over a range of working distances, as measured from a front of the reader; and electrical circuitry for producing electronic analog signals corresponding to the intensity of the light captured by the image sensor over the field of view, and for digitizing the analog signal. The imaging scan engine optionally supports an aiming light assembly for projecting a visible aiming light pattern, for example, a generally circular spot or cross-hairs for placement at the center of the symbol, to assist an operator in visually locating the symbol within the field of view prior to image capture, and a controller or microprocessor for controlling operation of the electrical components supported by the imaging scan engine, and for processing and/or decoding the digitized signal based upon a specific symbology used for the symbol.
It is therefore known to use the image sensor for capturing a monochrome image of the symbol as, for example, disclosed in U.S. Pat. No. 5,703,349. It is also known to use the image sensor with multiple buried channels for capturing a full color image of the symbol as, for example, disclosed in U.S. Pat. No. 4,613,895. It is common to provide a two-dimensional CCD with a 640×480 resolution commonly found in VGA monitors, although other resolution sizes are possible.
In some applications, the known laser and imaging scan engines are mounted in housings configured as stationary workstations, e.g., kiosks, or hands-free, fixed position, presentation scanners, having windows to which symbols are either presented, or across which the symbols are swiped. Some scan engines continuously capture and attempt to decode symbols without regard to whether or not a symbol is actually in the field of view. However, continuous, repetitive, flashing of bright light from the LEDs of the illuminating assembly in the case of the imaging scan engine, and continuous, repetitive, flashing of bright laser light from the laser in the case of the laser scan engine, consume and waste energy, degrade component lifetimes, and can be perceived as bothersome, distracting and annoying to the operators of the readers and to nearby consumers being served.
To alleviate these problems, the art has proposed periodically activating, i.e., waking-up, the scan engine with a few different approaches. For example, the scan engine can be operated with a very low duty cycle (about 10%). Thus, the scan engine wakes up for a very short period of time to scan the field of view and tries to detect a presence of a symbol therein. However, this creates sluggishness in the reader's performance and delays in decoding, which can be perceived as an engineering defect. Also, the distracting flashing laser light or illumination light is still present during the wake-up time period.
Another approach for the imaging engine tries to detect the symbol without energizing the illumination LEDs. However, under low ambient light conditions, for example, when a reader is located in a dark corner in a retail environment, the activation time will be slow since the exposure of the image sensor has to be very long in order to acquire an image of acceptable brightness for object detection. Again, the reader's performance is sluggish.
Still another approach for the imaging engine is to use the aiming pattern generated by the aiming assembly as auxiliary illumination for object detection. However, the engine activation is still somewhat slow (˜40 ms) due to the time necessary to perform some image processing to detect the symbol and set the image sensor's parameters for proper image acquisition for decoding. Also, since the aiming pattern is typically limited to a central area of the field of view, the scan engine can be activated only if the symbol is present within the center of the field of view, which contributes further to sluggish performance.
As exemplified by U.S. Pat. Nos. 5,280,162 and 5,449,891, a stand-alone dedicated object sensor can be installed in a reader for activating the scan engine only if an object bearing a symbol is detected within the active field of view of the scan engine. However, such installations are complex and expensive and require optical alignment procedures to be performed. The object detection field of the object sensor is not guaranteed to match the field of view of the scan engine. The object sensing feature is therefore not readily capable of being incorporated into new apparatus.